Workshop case study: Is this data correct? By Des Davies, Top Gear Motor Services
VEHICLE: FORD FOCUS 3 ECOBOOST 1.0 M1DD 2015 MILEAGE: 27,361 FAULT: P0137 SENSOR CIRCUIT LOW VOLTAGE SYMPTOMS: CHECK ENGINE LIGHT ON WITH NO OTHER ISSUES The car had previously been diagnosed by the Ford dealers, who had replaced a post-lambda sensor and within a few weeks, the check engine light had come back on again after using the vehicle on a few short journeys. I was asked to have a look to see what was causing the light to come back on. I first interrogated the customer to get information on the previous work history and any other faults or symptoms. I then test drove the vehicle – there were no symptoms of poor performance or misfires and the car ran well. I scanned the system and a P0137 fault code was stored. I checked the live data for fuel trims and lambda sensor output voltages: STFT 0.8%, LTFT 4.7% at idle STFT 3.1%, LTFT 4.7% at 3,000 rpm B1S1 O2 sensor current -0.02mA B1S2 O2 sensor output voltage 1.62V (that is a bit high!) I then used my 4-gas analyser to see what was happening at the exhaust tail pipe: CO 0.02%, HC 28ppm, CO2 15.20%, O2 0.10%, λ1.0 at idle CO 0.01%, HC 11ppm, CO2 15.20%, O2 0.2%, λ1.0 at 2,500 rpm. These looked rather good to me! Time now to get out the PicoScope to scope the oxygen post sensor output to see what was happening and to gather technical data from a well-known information resource for this vehicle. I scoped the signal wire, sensor ground and heater current and saw the incorrect waveforms in Figure 1. The signal wire was 1.5V, heater ground 14.7V and heater current 0A – something was not right here! I rechecked the technical data and the connection was correct, but something was very odd here.
I had another wiring diagram from Ben Martins at Pico who kindly sent me technical data from a different resource. This was different to the previous wiring diagram, but which one was correct? Or were they both incorrect? We must be so careful and vigilant to ensure we have the correct information to help us diagnose faults. This certainly is not the first time I have had problems with incorrect data from well-known resources, so we must be on our toes. All the years of experience and training does not prepare you for these problems, go by what you know and not what you do not know. Well, it was time to get back to basics and get out my trusted test light and multimeter. I disconnected the sensor connector and probed the sensor connectors with my ohmmeter to identify the heater circuit, which was around 8Ω. I then connected my test light to battery positive to find the sensor signal earth wire – when the test light illuminated, I had found the earth wire. I switched the ignition on and probed the harness side of the connector, still connected to battery positive, to find the heater earth wire. I then connected my test light to battery earth and one of the connector wires to find the heater feed, which was indicated by my test light illuminating. The remaining wire in the connector now had to be the sensor signal wire, I then had enough information to work on. The technical information and wiring diagram from Ben Martins at Pico had the correct technical circuit information for my vehicle. Now I could test the circuit knowing that I had the correct information. In Figure 2, you can see the results of my sensor connector wire tests.
Figure 1
The amps clamp was pulsing between negative and positive; the sensor output was 1.27V and then started switching for a few seconds before going back to 1.27V output. It was now time to check the integrity of the sensor and circuit before thinking of changing the lambda post sensor.
CASE STUDY
Something was not right with this waveform. Had I accidently got my scope on AC, or had my amps clamp gone AWOL? I checked it again, connected another amps clamp and no, this was the signal captured and all wires were connected correctly.
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